The present invention relates to video signal processing and picture display techniques in a display system such as television or the like, and particularly to a color display unit in which the quality (contrast ratio and color reproductivity) of a picture color-displayed on a plasma display panel is improved.
Recently, a plasma display panel is used for a display unit. This plasma display panel is designed so as to have two groups of linear or plane electrodes perpendicularly intersecting each other on two glass plates opposite to each other, and a voltage is applied across only a pair of specified ones of the electrodes of the respective groups so as to produce plasma at an intersection of the specified electrodes to thereby form a picture element of dot matrix display. In various flat display means, a plasma display panel is superior in realizing a large screen and is capable of performing full color display or high-speed gradation display. Accordingly, a plasma display panel is coming into practical use for a thin-type large-screen color display unit (for example, a display unit for a wall-mounted television set).
However, now a cathode-ray tube (hereinafter abbreviated to "CRT") is mainly used for a color display unit. Such a CRT has a display characteristic in which there is a relationship that when an input signal level and an output signal level (output luminance level) are represented by x and y respectively, y is proportional to the .gamma. (gamma)th power of x. Since the value of this gamma (.gamma.) is substantially equal to 2.2 as shown in FIG. 4, an original signal obtained by a camera or the like on an transmission side is corrected so as to have a reversed characteristic of .gamma.=0.45.
On the other hand, in a plasma display panel in which half tone display is performed by pulse number modulation, the value of gamma (.gamma.) is equal to "1" theoretically. That is, generally, a plasma display panel is only in either one of a turned-on state and a turned-off state, and half tone display is controlled by the number of times of turn-on of the plasma display panel in a short period (in one field period). That is, since the number of light emissions of a plasma display panel is proportional to the number of pulses supplied to the plasma display panel, the plasma display panel is subjected to the pulse number modulation with the number of pulses of the input video signal to thereby perform half tone display. Accordingly, if it is intended to perform display equivalent to that of a CRT, it is necessary to perform display after input signals (for example, R, G and B signals) are corrected so as to establish .gamma.=2.2.
However, in comparison with a CRT, such a plasma display panel used for a color display unit is dimmer, that is, lower in luminous efficiency; smaller in contrast ratio; and smaller in the number of display gradations so that a false outline is apt to be produced. If the number of display gradations is increased, the luminance level on the screen is reduced. Further, since the contrast ratio is small as mentioned above, there is a problem that the degree of color saturation of a picture is small, and hence the color reproductivity is poor. Furthermore, the low luminous efficiency, the small contrast ratio, and so on make it difficult to realize a large-screen wall-mounted television set.